Generally, fluidized-bed reactors are used in the conversion processes of hydrocarbons. Herein, a catalyst or similar particulates suited for heat exchange and fluidization is kept in a fluidized state by the upward flow of a gaseous hydrocarbon. Typically, the minimum fluidization flow is provided by means of a prefluidizing gas such as steam or recirculation of the product gas. In conventional fluidized-bed reactors operated with the linear flow rate of the medium adjusted close to the minimum fluidization flow rate, the particulate matter remains in the fluidized bed of the reactor, instead of becoming carried over from the reactor in significant amounts along with the hydrocarbon flow.
By contrast, at flow rates appreciably higher than the minimum fluidization flow rate, the upper surface of the fluidized bed becomes less defined, in fact, forming a zone in which the soids content deeses along the vertical axis. At sufficiently high flow rates this effect leads to a situation in which practically all the particulate matter will be carried over along with the hydrocarbon flow maintaining the fluidized state. Then, the solids must be separated from the hydrocarbon outlet flow leaving the reactor by means of cyclones and are recirculated either directly or via a regenerator back to the bottom section of the reactor. Such a system is called either a circulating fluidized bed (CFB), or analogously, a circulating fluidized bed reactor (CFBR), if a chemical reaction occurs in the suspended solids.